1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine and, more particularly, to an engine exhaust gas purification system of the type which uses an exhaust gas air-fuel ratio detector to control a secondary air supply so that the engine gases flow into an exhaust gas reactor at an air-fuel ratio which is optimum for the exhaust gas purification performance of the reactor. By the term "exhaust gas air-fuel ratio" (or "air-fuel ratio of exhaust gas") used herein, it is meant to say a ratio of the total of the amount of air introduced into the intake system of an engine and the amount of a secondary air introduced into the engine exhaust gases to the amount of a fuel supplied into the engine.
2. Description of the Prior Art
One type of the exhaust gas reactor used a catalyst of a type which is operative to simultaneously facilitate oxidization of carbon monoxide (CO) and hydrocarbon (HC) contained in engine exhaust gases and reduction of nitrogen oxides NO.sub.x) also contained in the exhaust gases. This type of catalyst has an inherent operating characteristic such that the catalyst exhibits its maximum performance only for an extremely limited range of the air-fuel ratio of the exhaust gases to be processed. For a different type of catalyst and for a different type of reactor (such as after-burner type reactor), there are different ranges of exhaust gas air-fuel ratio which are most suited for the optimum performances of the different type of catalyst and for the different type of reactor.
In general, it is known to use an exhasut gas air-fuel ratio detector to control a secondary air supply to engine exhaust gases. However, exhaust gas air-fuel ratio control generally used in the prior art, it is difficult to keep the exhaust gas air-fuel ratio within an optimum range throughout all the operating ranges of the engine. For this reason, it was impossible to enable the exhaust gas reactor to operate at its maximum exhaust gas purification performance throughout the operating range of engine.